The present invention pertains to a process for monitoring the tire pressure of the wheels of a vehicle by measuring, comparing and evaluating the rotating speeds of the individual wheels of the vehicle. The present invention also includes a circuit for carrying out the process.
Processes for recognizing the loss of tire pressure, which are based on the measurement and the comparison of the rotating speeds of the individual wheels of the vehicle, already are known from U.S. Pat. Nos. 4,876,528 and 5,192,929. In principle, the rotating speeds of the diagonally opposite wheels are added according to both of these patents, and the difference between the two sums is determined. If this difference is between 0.05% and 0.60% of the mean value of the two sums, the rotating speed of every individual wheel may deviate by at most 0.1% from the mean speed of all four wheels as long as a sufficient tire pressure is present. Loss of pressure is signaled in the case of greater deviations.
The accuracy of these prior art techniques leaves something to be desired. When a spare tire or "emergency tire" with a different diameter is mounted, the method can fail completely.
Other processes, based, in principle, on the same algorithm, are known from U.S. Pat. No. 3,581,277, JP 63-305011(A), and FR-25 685 19-A.
A process for evaluating the speed signals, which reflect the rotation behavior of the individual wheels of a vehicle, is known from DE 39 15 879-A1, in which a speed correction factor is determined for each wheel. The multiplication of the instantaneous wheel speed by the speed correction factor yields a common basic speed, which is derived from, e.g., the speed of the instantaneous slowest wheel. The wheel speed multiplied by the corresponding correction factor is then used as the basis for further signal processing, instead of the actual wheel speed. The process is used to determine the rotation behavior of a wheel within the framework of an antilock system.